1. Field of the Invention
The present invention relates to an optical information recording medium on which information can be recorded at a high density and high speed by using optical techniques such as irradiation of laser beams, a method for producing such an optical information recording medium and a method for recording and reproducing optical information with such an optical information recording medium.
2. Description of the Prior Art
An optical magnetic recording medium or a phase-changeable recording medium is known as a medium on which information can be recorded in large capacity and reproduced and rewritten at a high speed. Such a transportable optical recording medium is expected to be more important in a highly information-oriented society. The improvements in the function of applications and in the quality of graphic information require larger capacity and higher-speed recording or reproducing of the medium.
These optical recording media utilize a difference in the optical characteristics of the recording material created by locally irradiating the recording material with laser beams. For example, for the optical magnetic recording medium, a difference in the angle of rotation on a plane of polarization of a reflected light, which is created by a difference in the magnetized state, is utilized for recording. Furthermore, for the phase-changeable recording medium, an amount of reflected light in the crystalline state different from that in the amorphous state when light with a specific wavelength is used is utilized for recording. The phase-changeable recording medium is advantageous because erasing recorded information and overwriting information can be simultaneously performed by adjusting the output power of the laser, so that it is possible to rewrite information signals at a high speed.
FIGS. 5 and 6 show exemplary structures of layers of conventional optical information recording media. A resin such as polycarbonate, polymethyl methacrylate (PMMA) or glass can be used for a substrate 101. The substrate 101 may have a guide groove.
A recording layer 103 is formed of a material that has states having different optical characteristics and can change between the different states reversibly. In the case of a rewritable phase change type optical disk, a chalcogenide such as a material containing Te and Se as a main component can be used for the recording layer 103.
Protecting layers 102, 104 and 106 serve to protect the recording layer in such a manner that the recording layer material is prevented from being oxidized, evaporated or distorted. Furthermore, it is possible to adjust the absorption of the optical information recording medium or a difference in the reflectance between a recorded portion and a erased portion by adjusting the thickness of the protective layers. Thus, the protective layers also serve to adjust the optical characteristics of the medium. Moreover, a material for the protective layer is required to have good adhesiveness with a material forming the recording layer and the substrate, and good weather resistance so that the protective layer itself is not cracked.
Examples of the material for the protective layer include a dielectric such as a sulfide such as ZnS, an oxide such as SiO2, Ta2O5 or Al2O3, a nitride such as Si3N4 or AlN, a nitrogen oxide such as SiON or AlON, a carbide, a fluoride or the like, or suitable combinations thereof. Especially, ZnSxe2x80x94SiO2 has been generally used, because the layer composed of the material has less stress and good adhesiveness to the recording layer.
As shown in FIG. 5, in general, the protecting layers 102, 104 are formed on both sides of the recording layer 103. It has been proposed to produce a layered structure composed of two protective layers 102, 106 including different materials from the other, as shown in FIG. 6, so that the adhesiveness with the substrate and the characteristics in repetitive recording of information are improved.
A reflection layer 105 can be formed on the protecting layer 104 for the purpose of radiating heat and allowing the recording layer to absorb light effectively, but the reflection layer is not necessarily provided. The reflection layer is generally formed of a metal such as Au, Al, Cr or the like, or an alloy of these metals.
In general, an overcoating layer or a dummy substrate is located on the reflection layer 105 so that the optical recording information medium is not oxidized and dust or the like does not become attached to the medium, although those are not shown in FIGS. 5 and 6. The dummy substrate can be bonded with an ultraviolets-curing resin.
As a material for the recording layer, a Gexe2x80x94Sbxe2x80x94Te-based material has been investigated extensively, because the material is excellent in weather resistance and the characteristics in repetitive recording of information.
For example, Japanese Laid-Open Patent Publication (Tokkai-Sho) No. 61-89889 discloses a material for recording represented by Ge(1xe2x88x92x)Sb4xTe(1xe2x88x925x) (0 less than x less than 1). The material provides a substantial change of reflectance with a phase-change between an amorphous state and a crystalline state by irradiation of light beams. The material also can make the sensitivity to recording higher, because the material has a lower crystallization temperature to eliminate energy for crystallization. However, this investigation takes no account of a repetitive recording of information which involves a phase change between an amorphous state and a crystalline state.
Japanese Laid-Open Patent Publication (Tokkai-Sho) No. 62-53886 discloses a material for recording represented by (SbxTe(1xe2x88x92x))yGe(1xe2x88x92y) (x=0.05-0.7, y=0.4-0.8), and an example in which the irradiation of laser beams to a recording layer composed of the material can change the transmittance of the layer. However, this investigation takes no account of reversibility of the change and a repetitive recording of information.
Japanese Laid-Open Patent Publication (Tokkai-Sho) No. 62-196181 discloses a rewritable medium including a Gexe2x80x94Sbxe2x80x94Te-based material. A recording layer is composed of Ge23Sb46Te31 or the like, and has a protecting layer of SiO2, AlN or the like on the recording layer. The reflectance of the recording layer can vary reversibly by changing the power level of a semiconductor laser beam having a wavelength of 830 nm. However, this investigation takes no account of a repetitive recording of information and a preferable composition of the recording layer for irradiation of laser beams having a short wavelength of 680 nm or less.
The difference in an optical constant of a recording layer composed of a Gexe2x80x94Sbxe2x80x94Te-based material between an amorphous state and a crystalline state becomes narrower as the wavelength of the laser beams shortens for high-density recording. The narrower difference in optical characteristics such as reflectance makes the degree of signal change smaller, which restricts the density of recording.
Conventionally, a material having a composition ratio obtainable by adding some Sb to a stoichiometric composition ratio of Ge2Sb2Te5, or ones that are close to the composition ratio, has been used for a Gexe2x80x94Sbxe2x80x94Te-based material. It is believed that excellent characteristics in repetitive recording can be obtained, only because such a composition ratio is used for the recording layer. On the other hand, a composition ratio in the near side to GeTe on the GeTexe2x80x94Sb2Te3 line can enlarge the difference in an optical constant between the two states. However, such a composition ratio is not suitable for a rewritable medium, because the characteristics in repetitive recording of information deteriorate. The composition ratio far from the stoichiometric ratio is generally believed to cause the deterioration in repetitive recording.
Therefore, with the foregoing in mind, it is the object of the present invention to provide an optical information recording medium having sufficient difference in an optical constant with shortwave light beams and sufficiently excellent characteristics in repetitive recording so as to record and reproduce optical information effectively at a high density, and to provide a method for producing such an optical information recording medium and a method for recording and reproducing optical information with such an optical information recording medium.
An optical information recording medium of the present invention includes a recording layer having reversibly changeable optical characteristics. The recording layer contains at least three elements of Ge, Te and Sb, and the composition ratio of Ge, Te and Sb in the recording layer has numerical values that lie within the range represented by the area ABCDE in a ternary phase diagram of Ge, Te and Sb, where the points A, B, C, D and E are as follows:
A (Ge50Te50), B (Ge22.5Sb22.0Te55.5), C (Ge17.0Sb4l.5Te41.5), D (Ge48.0Sb26.0Te26.0), E (Ge65Te35),
The optical information recording medium also includes a diffusion preventing layer in contact with the recording layer. The diffusion preventing layer contains at least one compound selected from a oxide, a nitride, a nitrogen oxide, a carbide and a fluoride.
This makes it possible to provide a medium having a large difference in optical characteristics between an amorphous portion and a crystalline portion and excellent characteristics in repetitive recording.
In the optical information recording medium, the composition ratio of the recording medium is not close to the stoichiometric ones of Ge2Sb2Te5 but included in the above area. The combination of the recording medium with the diffusion preventing layer having the above ingredient makes a difference in optical characteristics large and characteristics in repetitive recording excellent. Thus, the present invention provides an optical information recording medium with the recording layer having a different composition ratio from ones that has been believed to be preferable, and the diffusion preventing layer having a material that cannot easily diffuse into other layers.
According to another aspect of the present invention, a method for producing an optical information recording medium includes a step of forming a recording layer having reversibly changeable optical characteristics by sputtering with a target containing at least three elements of Ge, Te and Sb. A composition ratio of Ge, Te and Sb in the target has numerical values that lie within the range represented by the area ABCDE in a ternary phase diagram of Ge, Te and Sb, where the points A, B, C, D and E are as described above.
The method also includes a step of forming a diffusion preventing layer in contact with the recording layer. The diffusion preventing layer is formed so as to contain at least one compound selected from the group consisting of a oxide, a nitride, a nitrogen oxide, a carbide and a fluoride.
This makes it possible to produce effectively a medium which has a large difference in optical characteristics between an amorphous portion and a crystalline portion and excellent characteristics in repetitive recording.
According to another aspect of the present invention, a method for recording and reproducing optical information utilizes an optical information recording medium including a recording layer having reversibly changeable optical characteristics, and a diffusion preventing layer in contact with the recording layer. The recording layer contains at least three elements of Ge, Te and Sb, and a composition ratio of Ge, Te and Sb in the recording layer has numerical values which lie within the range represented by the area ABCDE in a ternary phase diagram of Ge, Te and Sb, where the points A, B, C, D and E are as described above. The diffusion preventing layer contains at least one compound selected from an oxide, a nitride, a nitrogen oxide, a carbide and a fluoride.
The method utilizes laser beams focused on a microspot on the optical information recording medium to record or reproduce optical information. A power level of the laser beams is fluctuated between a first power level P1 and a second power level P2 to introduce a change of an optical state of the recording layer, that is, a change of the optical information. The change of the optical information is selected from recording, erasing and overwriting. On the other hand, the optical information is reproduced with the laser beams of a third power level P3.
The first power level P1 is an amorphous state formation level which allows a portion in the recording layer to reversibly change to an amorphous state by irradiation of the laser beams. The second power level P2 is a crystalline state formation level which allows a portion in the recording layer to reversibly change to a crystalline state by irradiation of the laser beams. The third power level P3 is a reproduction level which is lower than the power levels P1 and P2, and does not influence the optical state of the recording layer and provides a sufficient reflectance so as to reproduce the optical information.
This makes it possible to record, reproduce and rewrite optical information effectively at a high density by utilizing an optical information recording medium which has a large difference in optical characteristics between an amorphous portion and a crystalline portion, and excellent characteristics in repetitive recording.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.